1. Field of the Invention
The present invention relates generally to the communication of digital signals and more specifically to receiver synchronization of multiple diversity channels in a digital communication system.
2. Description of Related Art
In digital communication systems, digital symbols, such as binary xc2x11 values, are transmitted as waveforms through a channel from a transmitter to a receiver. The term xe2x80x9cchannelxe2x80x9d is used here in a general sense, and refers to any medium through which signals are transmitted. For example, a channel may be a radio environment, a copper wire, an optical fiber, or a magnetic storage medium. In each case, the signal received at the receiver differs from the signal transmitted by the transmitter due to the effects of transmission through the channel. The received signal often includes noise and interference from other signals which diminish the quality of the signal and increase the probability of transmission errors.
In wireless communications systems in particular, a phenomenon known as Rayleigh fading may cause highly localized signal losses of 40 dB or more due primarily to signal path differences. In order to overcome Rayleigh fading, it is known to employ a plurality of antennas at the receiver in a technique known as spatial diversity. When the receiver antennas are physically separated by a sufficient distance, the signals received by the antennas exhibit uncorrelated Rayleigh fading. The signals received by the antennas are referred to as xe2x80x9cdiversity signals,xe2x80x9d and the antennas are referred to as xe2x80x9cdiversity antennas.xe2x80x9d The diversity signals are combined at the receiver to produce a more robust, intelligible signal.
Closely spaced antenna elements may also be used, as in a phased array, to provide array gain, even though diversity gain may be thereby reduced or eliminated. It may be preferable to apply beamforming to phased array signals prior to demodulation.
At the receiver, signal preprocessing operations such as filtering, amplification, and possibly mixing are performed on the signal prior to demodulation. The signal preprocessing operations may also include sampling and quantizing the received signal to obtain a sequence of received data samples. Following such signal pre-processing, the received signal is demodulated and converted to analog for output.
In most digital communication systems, synchronization (or xe2x80x9csyncxe2x80x9d) signals sent by the transmitter assist the receiver in demodulating the received digital signals. The receiver compares the received signals with copies of the known sync signals to determine the bit or symbol timing, to determine frame timing, and possibly to estimate the channel response. The symbol timing indicates the best place to sample the received signal and the frame timing indicates where the start of a new frame occurs. If oversampling is performed, timing indicates which sampling phase to use when decimating the oversampled data.
With conventional synchronization methods, timing is determined by finding a sampling phase which maximizes the signal strength of the desired signal. Typically this is done by correlating the received signal to the sync signal and using magnitude squared correlation values as indications of signal strength.
Unfortunately, the received signal includes an impairment signal that prevents perfect recovery of the transmitted digital symbols. If the impairment is Additive White Gaussian Noise (AWGN), then the conventional strategy of maximizing signal strength described above also maximizes signal-to-noise ratio (SNR) at the input of the demodulator. If the impairment consists of other signals, such as co-channel interference or adjacent channel interference, then the input signal to impairment plus noise ratio (SINR) can be maximized according to the method discussed in U.S. Pat. No. 5,406,593 to Chennakeshu et al.
When multiple receive antennas are employed for spatial diversity, the conventional approach is to synchronize each diversity signal separately, as discussed in U.S. Pat. No. 5,406,593. This optimizes the SNR or SINR on each diversity channel. This approach makes sense with conventional diversity combining in which no interference cancellation is performed, as the demodulator output SINR is, at best, the sum of the SINRs of the different diversity channels. However, when interference cancellation is performed at the receiver, maximizing the SINR on each antenna is not necessarily the best strategy. Rather, it may be advantageous to coordinate the interfering signals on different antennas in time, so that the interference components of the various signals will cancel one another when the diversity signals are combined. This is something separate channel synchronization cannot guarantee. Thus, there is a need for a method and apparatus capable of jointly synchronizing multiple receive channels to maximize the performance of an interference canceling detector.
It is, accordingly, a primary object of the present invention to provide an apparatus for joint synchronization of multiple receive channels.
In accordance with the present invention, an apparatus for joint synchronization of multiple receive channels is provided. The apparatus includes means for receiving signals, means for preprocessing received signals, means for joint synchronization of the preprocessed signals, and means for canceling interference in the synchronized signals, wherein the data contents of the received signals are determined after cancellation of the interference.
A method of jointly synchronizing multiple receive signals is further provided. In accordance with the present invention, a sampling phase offset is selected for each diversity signal such that the SINR of the combined receive channels is maximized.
These and other objects of the invention, together with features and advantages thereof, will become apparent from the following detailed description when read with the accompanying drawings in which like reference numerals refer to like elements.